Microsoft Corp.MSFT-0.38% racked up a whopping $8.5 billion phone bill to buy Skype SARL even though there were no signs of other serious bidders for the provider of free online video and voice chats, as the software giant moved aggressively to ramp up its growth.

Microsoft made an unsolicited bid for the Internet company last month and clinched its deal late Monday, nixing a planned Skype public offering and short-circuiting any talks with competitors such as Google Inc. and Facebook Inc.

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Steve Ballmer, Microsoft's chief executive, defended the price in an interview, saying the deal—the biggest in his company's 36-year history—will let Microsoft "be more ambitious, do more things."

While Facebook, Google and Cisco Systems Inc.CSCO0.42% had shown interest in Skype, Microsoft was by far the most determined buyer, people familiar with the matter said.

The price tag—three times what Skype fetched 18 months ago—is a sign of just how hungry Microsoft is for growth opportunities—especially in the mobile-phone and Internet markets. Those missed opportunities are increasingly worrisome to Microsoft investors as traditional profit engines, like its Windows software, are showing signs of slowing.

The Skype deal is a gamble by Mr. Ballmer that he can succeed where those that have gone before him have failed: using the phone-and-video-calling service to make money. Microsoft's ambitious goal is to integrate Skype into everything from its Xbox videogame console to its Office software suite for businesses.

Microsoft also hopes Skype can jump-start its effort to turn around its fortunes in the mobile-phone market, an area where it has lagged far behind Apple Inc. and Google. Phones running Microsoft software were just 7.5% of the smartphone market last quarter, according to Comscore Inc.

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The Skype purchase comes as the technology industry's momentum is increasingly being fueled by consumers, with the explosive rise of social network Facebook, now at more than 600 million global members, and devices such as Apple's iPad and iPhone reshaping the cellphone and computer markets.

That has pushed many big tech companies that had largely relied on businesses for growth—from Dell Inc. to Cisco—to seek ways into consumer technologies.

Some of those moves haven't paid off, however. Cisco, for example, recently shut down the division that made its Flip video cameras, just two years after acquiring the business.

Whether Microsoft can make a Skype acquisition work—especially at such a rich price—is a question mark. EBay Inc.EBAY-0.88% bought Skype in 2005 for around $3.1 billion but took a $1.4 billion charge for the transaction in 2007 after it failed to produce hoped-for synergies.

EBay decided to shed the business, and sold a 70% stake in Skype to a group of investors led by private-equity firm Silver Lake Partners about 18 months ago. The deal valued all of Skype at a $2.75 billion.

Mr. Ballmer said Microsoft and Skype have far more in common than Skype had with eBay since both companies are in the "communications business." He said communications technologies have been "the backbone" of Microsoft's growth in recent years and that Skype has "built a real business," with more than $860 million in 2010 revenue.

"I think our case for why to bring this together comes from a very different place," he said.

Overall, Skype has more than 170 million active users and 207 billion minutes of voice and video conservations flowing through its service. But despite its widespread use, it has been slow to convert users into paying customers and generate meaningful profits. It had a net loss of $7 million last year.

Skype lets people make free phone calls between computers or pay pennies-a-minute to make overseas calls from traditional phones, and increasingly it's being used in the form of an app on smartphones to avoid paying higher fees to wireless carriers for phone calls.

The acquisition comes three months after Microsoft struck an ambitious partnership with cellphone maker Nokia Corp.NOK0.28% under which the Finnish company will rely on Microsoft software to power its smartphones.

Nokia, which is now heavily exposed to the decisions Microsoft makes regarding its mobile software, declined to say whether it had received a heads up on the deal. "Skype has been available to Nokia users for a number of years already, and we certainly understand the value of the asset that Microsoft has just acquired," a spokeswoman said. She said the company will watch to see how Microsoft makes use of Skype.

Mr. Ballmer said Microsoft would continue to make Skype available for Apple's gadgets and devices running Google software.

History of Skype

Tony Bates, Skype's chief executive, said Microsoft will help Skype expand its audience into the billions, from hundreds of millions of users. "I think our combined assets really accelerates and extends that," he said.

Skype filed for an IPO last August and had since held discussions with various companies about potential partnerships, said people familiar with the matter. Microsoft was exploring partnerships with Skype, Mr. Ballmer said, but quickly concluded an acquisition made the most sense. In early April, he made an unsolicited offer to Silver Lake, which owns 39% of the company.

Microsoft at that point was the only serious bidder, according to people familiar with the matter, but the IPO alternative was credible enough to help Skype drive a harder bargain.

"Microsoft put forth a very compelling offer," said Egon Durban, a partner at Silver Lake who negotiated the deal.

The $8.5 billion price tag for Skype surprised some industry executives and investors Tuesday, sending Microsoft's shares down 16 cents to $25.67 on Nasdaq. Mr. Ballmer said the deal will add to Microsoft earnings during the first year after it closes, though that calculation excludes certain items such as amortization. Microsoft expects the deal to close by the end of 2011.

Meg Whitman, eBay's former CEO who faced criticism for acquiring Skype, said she was gratified to see the company bought for such a high number. She said Skype is at the center of a "sea change" in how people communicate with each other.

"Is Skype worth $8.5 billion? I don't know, but it depends on how big the platform grows," said Ms. Whitman. EBay, which initially lost money on the Skype acquisition, will end up with a profit of about $1.4 billion.

One point in Microsoft's favor: the company plans to purchase Luxemburg-based Skype with cash held overseas, money that it could not otherwise bring back to the U.S. without paying repatriation taxes of more than 30%. The majority of Microsoft's roughly $50 billion in cash is held overseas.

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